1. Field of the Invention
The present invention relates to a flat color cathode-ray tube (CRT), and more particularly, to a tension mask for a CRT having a color selection function and a tension mask frame assembly.
2. Description of the Related Art
In typical color CRTs used for computer monitors and televisions, three electron beams emitted from an electron gun pass through beam passage holes of a mask for color selection and land on red, green and blue phosphors of a phosphor layer formed on the screen of a panel to excite the phosphors, thereby forming an image. In such conventional color CRTs, representative masks for color selection are dot masks employed in computer monitors and slot (or slit) masks employed in televisions. Such dot and slot masks are designed to have a curvature corresponding to the curvature of a screen because the screen is formed to have a predetermined curvature taking into account landing of deflected electron beams.
The above-described masks are formed by etching a thin plate having a thickness of 0.1-0.25 mm (millimeters) to form a plurality of beam passage holes and shaping the thin plate to have a predetermined curvature. If the conventional masks do not have at least a predetermined curvature, their structural strength is weak, so the masks can be plastically deformed during fabrication of CRTs. Consequently, the masks cannot perform their inherent color selection function. Recently, CRTs tend to be flat, and the conventional flat CRTs impose many restrictions on the fabrication of completely flat CRTs.
U.S. Pat. No. 3,638,063 by Tachikawa et al. for Grid Structures for Color Picture Tubes discloses an aperture grill-type mask for preventing a doming effect and achieving flatness. In this aperture grill-type mask, strips made of a thin steel plate having a thickness of 0.1 mm (millimeter) are not coupled, and only both ends of each of the strips are supported by a frame, so the strips are individually caused to vibrate even by slight impact to thus cause howling of a screen.
U.S. Pat. No. 4,942,332 by Adler et al. for Tied Slit Mask for Color Cathode Ray Tubes discloses a slot-type mask. A slot-type mask includes a plurality of strips separated from each other by a predetermined distance to thus form slots and tie bars coupling the strips. A long side of the mask is fixed to a support member.
In this mask, the tie bars coupling the strips serve to reduce howling of a screen arising from the vibration of the mask due to external impact but do not make a great contribution to reproduction of Poisson contraction occurring when tension is applied to the mask. In other words, when tension within an elastic limit is applied to the mask in a vertical direction, the mask expands in the vertical direction and contracts in a horizontal direction. Accordingly, slots at both end portions of the mask are displaced, and the edges of the short sides of the mask move outward due to the tie bars when the mask thermally expands.
In addition, when tension is applied to the strips, the slots defined by the strips and the tie bars have a wider horizontal width at their middle than a horizontal width at their upper and lower portions due to Poisson contraction of the strips.
Such nonuniformity of a strip width causes collimation of strips to be poor and furthermore causes collimation of red, green, and blue strip patterns to be poor during exposure of a phosphor layer, so the phosphor layer cannot be formed in a uniform pattern.
Such change in slots occurring when tension is applied to strips is greater in a long slot-type mask. Such a long slot-type mask is disclosed in U.S. Pat. No. 4,973,283 issued to Adler et al. for Method of Manufacturing a Tied Slit Mask CRT. U.S. Pat. No. 4,973,283 restricts a mask such that the result of dividing the vertical pitch of a slot by the horizontal pitch of the slot should be greater than 16 (vertical pitch/horizontal pitch>16) in order to minimize the quantity of movement of a slot at the edge of an effective surface. When the slots get longer as described above, the strips defining the slots also get longer. As a result, excessive Poisson contraction of the strips is induced, and the width of each slot at its middle portion is wider than the width of the slot at its upper and lower portions.